Photographic paper is coated with a polyolefin layer or layers preferably by extrusion from a hot melt as is known in the art.
The polyolefin can be extruded with a wide temperature range, i.e., 150.degree. C.-350.degree. C. through a coat hanger die into the nip formed by a metallic chill roll and an elastomeric pressure roll. The pressure roll causes a nip pressure of typically 17,500 Pa-m to 175,000 Pa-m, where the nip is defined as the point where pressure is applied, to the point where pressure is relieved, which forces the polyolefin into the interstices of the porous paper forming a mechanically interlocked polymer/paper composite.
The coated paper is then cooled on the chill roll which is operated at a temperature of between 4.degree. C. to about 32.degree. C. and conveyed to the stripping point of the chill roll where the paper is removed by tension, and wound into a roll or conveyed to the next coating operation.
In the current configuration used in the industry, the temperature of the chill roll must be warm enough to allow the polymer to penetrate the pores of the paper in the nip. If the chill roll is too cold, the polymer will be cooled below its melting point before it has an opportunity to penetrate the pores of the paper, and poor bonding will result. This will deleteriously affect product quality, often resulting in the penetration of photographic processing chemicals between the paper/thermoplastic interface which will result in edge stains on the finished photographs.
Alternatively, if the chill roll is too warm, the thermoplastic will act as an adhesive, and will not be stripped off the chill roll properly, often resulting in a web break which decreases productivity. Thus the range of chill roll temperatures is dictated by the conflicting needs of good bonding and high productivity. In order to increase productivity and get good bond, it is common practice to increase the polymer melt temperature, which improves the bond. However, an increase in the polymer melt temperature will often result in curtain instability, and cause the polymer to degrade in a chain scission reaction (Billmeyer, Textbook of Polymer Science, Wiley Interscience publication, 1984, pp 141-143). This results in impurities and a general reduction in the polymer physical properties.